Storm Forecast
Valid: Thu 03 Sep 2009 06:00 to Fri 04 Sep 2009 06:00 UTC
Issued: Wed 02 Sep 2009 19:42
Forecaster: GATZEN
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A level 1 was issued for Poland, eastern Germany, southern Baltic Sea region mainly for severe winds and tornadoes, and to a lesser extent for large hail.
A level 1 was issued for Southeastern Germany and the Czech Republic mainly for severe winds.
SYNOPSIS
Along the southern flank of an intense long-wave trough over north-western Europe, a 40 m/s mid-level jet streak spreads into central Europe. Downstream, the delta of this jet moves northward across Scandinavia. While a tongue of moist air masses is advected northward ahead of the cold front, drier air spreads into the British Isles and northern France. Rich low-level moisture is present over the Mediterranean.
DISCUSSION
Poland, eastern Germany, southern Baltic Sea region
Underneath the nose of the strong curved mid-level jet, models indicate a cold front that propagates across Germany and Poland. Rich low-level moisture with dew points in the mid 10s is forecast to spread into Poland and the surroundings, and some instability is likely due to relatively steep lapse rates in the warm air mass. In the wake of the cold front, rather moist low-level air mass may also destabilize given strong lift along a trough axis moving across northern Germany, leading to ageostrophic winds and some low-level warm air advection combined with strong lift.
Current thinking is that numerous showers and thunderstorms will spread eastward in the range of the cold front. High coverage of storms may lead to clusters especially over Germany where the forcing is strongest. This may be associated with rather cool low levels and weak low-level buoyancy. But given strong deep layer vertical wind shear, more discrete convective cells may also form, and diabatic heating may enhance the low-level buoyancy locally. This is also expected over Poland, where discrete cells are more likely. These thunderstorms will be capable of producing tornadoes given strong low-level vertical wind shear and favourably veering profiles. The potential of strong tornadoes seems to be centred in the range of a 20 m/s low-level southerly jet streak moving across Poland during the day. Additionally, severe wind gusts are not ruled out as well as locally large hail in regions with stronger diabatic heating. Storms will likely spread across the southern Baltic Sea region while the potential of tornadoes will gradually decrease during the evening and night hours.
In the wake of the cold front, ageostrophic winds may lead to some warm air advection over northern Germany, where strong lift may lead to destabilization. As vertical wind shear is strong, thunderstorms may organize and may produce some hail and wind gusts. A few mesocyclones capable of producing large hail and tornadoes are not ruled out, but current thinking is that cold air advection will lead to low-level stabilization rather quickly in the afternoon hours, limiting the severe potential.
Southern Germany, northern Austria, Czech Republic
Ahead of the cold front and a following trough axis, moist low-level air mass will likely destabilize during the day. Given strong low-level forcing, convection will likely develop and organize given strong vertical wind shear. Tornadoes are not ruled out, but the potential seems to be weaker as further north given rather poor veering profiles and increasing cold air advection at low-levels in the wake of the cold front that moves eastward quickly.
North Mediterranean
Given steep mid-level lapse rates and rich low-level moisture, CAPE is expected over a large region. However, differential anticyclonic vorticity advection becomes likely during the day at the southern flank of the mid-level jet streak. Some warm air advection is also forecast that may compensate this, and local thunderstorms are not ruled out. As deep layer vertical wind shear in the northern portions is strong, storms may organize capable of producing large hail. Strong to severe wind gusts are not ruled out.
